1. Field of the Invention
The invention relates to a polarization rotator and crystalline quartz plate for use with an optical imaging system having several imaging optical components arranged in succession along an optical axis, means for creating radially polarized light arranged within that region extending up to the last of said imaging optical components, and a crystalline-quartz plate employable on such a system.
2. Description of the Related Art
German laid-open publication DE 195 35 392 A1 discloses an optical imaging system of said type in the form of a microlithographic projection exposure system having, e.g., an i-line mercury discharge lamp as a light source. Said system's employment of radially polarized light for exposing wafers was intended to improve the coupling of said light into the layer of photoresist, particularly at very large angles of incidence, while simultaneously achieving maximum suppression of any standing waves that might be caused by reflections at the inner and outer interfaces of said photoresist. Various types of radial polarizers that employ birefringent materials were mentioned as prospective means for creating radially polarized light. That radial polarizer chosen was arranged within that region that followed said system's final phase-correcting or polarizing optical element in the optical train in order that the degree of radial polarization attained prior to incidence on said wafers would remain unchanged. In the event that a catadioptric optical system were employed as said system's projection lens, the radial polarizer involved should be preferably arranged, e.g., following said optical system's final deflecting mirror. Otherwise, it might be arranged, e.g., within the preceding illumination system of the projection exposure system.
Radially polarized light, i.e., light that is linearly polarized parallel to its plane of incidence on an interface, is, in general, preferable in cases involving imaging optics, e.g., the imaging optics of microlithographic projection exposure systems, since radially polarized light allows employing highly effective antireflection coatings on their imaging optical components, particularly their lenses, which is a matter of major importance, particularly in the case of microlithographic projection exposure systems with high numerical apertures and at short wavelengths, e.g., wavelengths falling in the UV spectral range, since there are few coating materials that are suitable for use in that spectral range. On the other hand, tangentially polarized light, i.e., light that is linearly polarized orthogonal to the plane of incidence of an imaging light beam on the respective interfaces of the lenses, or similar, involved, should preferably be employed for illumination in order to allow creating the best possible interference-fringe contrasts when imaging objects on, e.g., wafers. In order to allow same, the older German patent application 100 10 131.3 proposed employing a tangentially polarizing element arranged in the vicinity of a pupillary plane of the projection lens, or within the illumination system that precedes same in the optical train that may be assembled from segmented birefringent plates instead of the radial polarizer of German patent disclosure DE 195 35 392 A1.